This invention relates to apparatus for calibrating air flow controllers in heating and air conditioning ventilation systems. More particularly, the invention relates to apparatus which simulates ventilation system pneumatic signals for pre-calibrating or re-calibrating to engineering design specifications an air flow controller in a heating and air conditioning ventilation system.
Among the components of a typical heating, ventilation, and cooling (HVAC) system are ducts, fans, dampers, and thermostats. The fans blow air through the ducts to ventilate rooms of a building. The dampers control the volume of air flowing through individual ducts. A damper is a movable plate contained in a duct. A damper actuator having a piston arm driven by an air cylinder connects to and moves the damper in response to a signal from the controller. A thermostat monitors the air temperature and communicates with a damper controller. The controller responds to the air temperature signal from the thermostat and operates the damper actuator to further open or close the damper.
One known control system for heating, ventilating and cooling is a variable air volume system which uses air pressure signals to control dampers for directing variable volumes of air flow through a duct network to different rooms or zones. A zone is a group of rooms handled by one thermostat. A thermostat in any given room or zone generates an air pressure signal which communicates through a tube or air pipe to a controller. This signal reflects the temperature sensed by the thermostat. The controller responds to the air pressure signal and operates an actuator to open or close the damper in the ventilation duct leading to the room.
A standard supply of air under pressure is input to each thermostat. Typically this main air supply is about 20 pounds per square inch (psi). Typically the output pneumatic signal from a thermostat varies from about 3 psi to about 17 psi. Output signals may be as low as 0 psi or as high as 20 psi. However, signals at these extremes may indicate there is a more serious problem with the air flow system since air control equipment reaches its maximum response before the thermostat signal reaches an extreme level. In typical installations, a duct damper opens for maximum air flow when the thermostat is generating an output signal of about 15 psi. The damper closes to a minimum air flow position when the thermostat signal is about 9 psi. This position is reached when the thermostat reaches its setpoint, i.e., the desired temperature for the room.
Generally, a damper closes no further than a position which allows a minimum air flow through the duct. Also, the damper opens no further than a position which allows a maximum amount of air flow. These minimum and maximum air flow rates are determined when designing the HVAC system. The minimum and maximum positions for the damper are set by calibrating the damper controller during system installation (or if necessary or desired, recalibrated after the HVAC system is operational.) Thus, a damper in the minimum air flow position restricts most, but not all, of the air flow in a duct. Should more air flow be required in a duct, the damper in the duct opens in response to a signal from the damper controller. The damper takes intermediate positions between the minimum and maximum air flow position, depending on the ventilation requirement of the room to which the air is flowing. As ventilation requirements change, the damper opens and closes in response.
The controller stands between the thermostat and the damper actuator. The controller generates a branch signal based upon the thermostat signal and the differential pressure of air flowing through the duct. The branch signal is the pneumatic signal from the controller to the damper actuator. The differential pressure in ducts is obtained by means of air pressure sensing tubes positioned in the ducts. One tube opens towards the air flow to sense the impact pressure while the other opens the air flow to sense the static pressure. The differential pressure provides an indication of air flow rate in the duct.
The controller translates the thermostat signal and duct differential air pressure into a relative damper movement. After calibration, the controller will not direct the damper to open any further than the maximum air flow position, even if the thermostat signal is requesting additional air flow. This is true for minimum air flow as well.
Pneumatic damper actuators operate using the branch output signal communicated from the controller. The branch signal enters an air cylinder which pushes a piston connected to the damper. The typical operating range for these air cylinders is 8 to 13 psi. Over that range, the damper moves from its minimum position to its maximum position.
A damper actuator unit may also include an auxiliary heating capability which is activated when the thermostat signal is reflecting a temperature relatively lower than the desired temperature. The heating capability is from a warm water coil, electric strip heater, or a warm air duct. The air flow into a room which is cooler than desired may be warmed by passing over the warm water coil, electric strip heater, or by blending warmer air into the air flow.
Shown in Table I is a comparison of the thermostat output pressure to temperature in degrees Fahrenheit for a typical installation. The thermostat output signal changes approximately 2.5 psi per degree change in air temperature. In this example, the mid-range thermostat reading of 72.degree. F. has an output signal of approximately 8.5 psi.
TABLE I ______________________________________ AIR FLOW, THERMOSTAT AND TEMPERATURE COMPARISON THERMOSTAT APPROXIMATE DESCRIPTION OF OUTPUT SIGNAL TEMPERATURE AIR FLOW (PSI) (Degrees F.) ______________________________________ 17 16 Maximum 15 75.degree. 14 13 12 11 10 11 9 Minimum 8 72.degree. No 8 Heat 7 Minimum 6 70.degree. (with auxiliary 5 heating) 4 3 ______________________________________
Signals for temperatures over 75.degree. are effectively requiring additional cooling for the room. Because the damper controller is set to stop opening the damper at a specified flow rate, the room continues to receive the maximum flow rate--the rate it was receiving when the thermostat reached 75.degree.. The same is true for heating. Below 70.degree. F. the room is receiving minimum air flow with auxiliary heating, the same as it received when the thermostat was reading about 71.degree..
The mid-range temperature is a target or desired room temperature. The temperature reading on the thermostat which corresponds to the mid-range output pressure may be adjusted by the technician. A mid-range of 76.degree. F. would have a range of about 74.degree. to 79.degree. F.
For a typical HVAC system operating in a cooling mode, design engineers determine the maximum and minimum air flow rates to accomplish maximum and minimum cooling. As explained, the maximum flow rate for cooling a room has the damper to the room open as much as necessary to reach the maximum flow rate. The minimum air flow occurs when the room is cool; the damper is then positioned in the duct to restrict as much air flow as necessary. For temperatures between the maximum and minimum temperatures, the damper takes intermediate positions. For a warm room not at the maximum temperature, the damper is positioned at an intermediate position close to the maximum air flow position. As the room cools, the pneumatic thermostat output signal changes. The controller responds and signals the actuator to close the damper and restrict air flow through the duct to the room. Each controller must be set or calibrated individually according to specifications provided by the HVAC engineers.
The present method of calibrating the controllers in an HVAC system is time consuming and subject to error. In general, the range of air flows desired in each duct is determined by reference to engineering blueprints. Then, using standard charts, the differential air pressures corresponding to the specific minimum and maximum flow rates for the controller are ascertained. These charts are semi-log graphs of the pressure drop (or differential pressure in inches of water) plotted against air flow (in cfm) for different size ducts. The ducts are identified by an assembly size number which corresponds to the duct cross-section. The plot results in a series of slanted parallel lines for different duct sizes. The chart is typically found in installation and maintenance manuals published by the HVAC system manufacturer. Also the differential pressure/flow rate chart is frequently affixed to the damper actuator housing or the duct so that the information is readily available to the technician calibrating the controller.
The controllers in HVAC systems normally have dials or equivalent features to adjust the minimum and maximum flow positions of the dampers controlled by the controller. For example, the "TITUS" air flow controller made by the Environmental Elements Corporation has one dial for adjusting the high flow damper position and a separate dial for adjusting the low flow position.
In balancing an HVAC system for cooling using a controller such as the TITUS controller, the individual controllers in the system are set in sequence. The technician sets the thermostat associated with any given controller to be calibrated at its highest temperature. An air pressure gauge such as a MAGNEHELIC gauge manufactured by Dover Instruments Inc. is then connected to the "high end" and "low end" test pressure fittings in the duct controlled by the controller. These fittings connect to the air pressure sensing tubes in the duct to obtain the high impact pressures and low static pressure available with such tubes. The resulting differential air pressure reflects the flow rate in the duct.
With the HVAC system in operation, the minimum air flow dial on the controller is adjusted so that the branch output pneumatic signal from the controller to the damper actuator causes the damper to move toward the minimum air flow position. The technician watches the differential pressure reading on the MAGNEHELIC or equivalent gauge to determine when the damper is positioned for minimum air flow. Since the air system is dynamic, the reading may fluctuate, but the technician attempts to have the desired differential pressure at the middle of the needle fluctuations on the gauge. The technician must wait for the damper actuator to respond to the changed controller setting before evaluating the differential pressure reading on the gauge. Based on this evaluation, further controller setting adjustments may be necessary.
Once the low flow position is set, the technician sets the maximum flow position. Returning to the thermostat, the technician sets the thermostat to its lowest temperature setting. He returns to the controller in the ceiling area and adjusts the maximum air flow dial so that the damper moves towards the maximum air flow position. Again, the system must respond to the changed controller dial before the technician evaluates the differential pressure reading. The technician checks the differential pressure gauge to see if the differential pressure is that which he determined was correct for the maximum air flow. If the differential pressure is not correct, then the controller dial is further adjusted.
This procedure is then repeated several times, since a change in the setting of one of the dials normally impacts the setting on the other dial. However, the adjustment may be in vain, since improper calibration of an upstream controller may result in insufficient air capacity to reach the necessary maximum air flow for the controller being calibrated.
It will be apparent that the above calibration procedure requires careful and patient attention by the technician in implementing the procedure. Any lack of care or patience can readily lead to erroneous settings and controls. Improperly calibrated controllers which permit maximum air flow greater than the design specification results in system operation inefficiencies, insufficient air for downstream controllers and improper cooling and heating of the control rooms. Also, miscalibrated controllers which restrict air flow beyond the minimum specification create problems impacting the performance of other controllers. System operation inefficiencies and improper cooling and heating of the control rooms can also result.